The present invention relates to a biaxially oriented polyester film for floppy disks which is excellent in dimensional stability and flatness.
A polyester film is used for various purposes and it is especially suitable as a base film for a floppy disk.
With the increasing demand for higher-density disks, a base film for a floppy disk has been required to reduce the dimensional change of the recording medium in a severe environment. To meet such demand, a method of producing a polyester film by the relaxation and heat treatment of a biaxially oriented and heatset film in the off-line is generally adopted to reduce the dimensional change of a polyester film (e.g., Japanese Patent Publication No. 10769/1983, and Japanese Patent Publication Nos. 15927/1982 and 127233/1984).
Although this method can produce a film with a small dimensional change, the low productive efficiency such as workability disadvantageously leads to rise in cost. In addition, the flatness of the film is lowered. Thus, the improvement of this method is demanded.